Both vak and shrotra concern sound, and a system less committed to a strict receiving/acting divide might have folded them together as two aspects of one auditory faculty. Samkhya does not. Shrotra belongs among the five Jnanendriyas, the organs that only receive; vak belongs among the five Karmendriyas, the organs that only act, and nothing about their shared subject matter, sound, overrides that classification. The distinction the system is protecting is functional rather than topical: what matters for Samkhya's placement of a tattva is whether it takes information in or puts effect out, not what kind of information or effect is involved.
Vak is produced, along with pani, pada, payu, upastha and manas, from the sattvic aspect of ahamkara, the same source that produces the five Jnanendriyas, which is why all eleven organs share a common origin despite splitting cleanly into receptive and active halves once produced. Vak's specific action, articulate speech, is treated as the paradigm case of the Karmendriyas' work generally: a real effect on the world, distinct from and downstream of the perception and judgment that precede it in the classical sequence of sensing, coordinating, claiming and deciding.
Vedic literature had already personified speech as the goddess Vac well before Samkhya's systematic philosophy took shape, and the tattva vak inherits that older reverence for speech as a power rather than a mere physiological output, even though the classical Samkhya texts themselves are more interested in vak's formal place among the twenty-five tattvas than in celebrating it.